Sersic showed that the exponential and de Vaucouleurs profiles may be seen as special cases of a dis

Sersic showed that the exponential and de Vaucouleurs profiles may be seen as special cases of a dis

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Sersic showed that the exponential and de Vaucouleurs profiles may be seen as special cases of a distribution now bearing his name: Here, the parameter n is a concentration index. This function has often been used when dealing with poorly resolved galaxies, or when it is not clear that either of the two kinds of fits just discussed is appropriate. Recent work suggests that it may be a generally useful function which in fact describes galaxy spheroids better than the classic de Vaucouleurs or King laws. There is a major limitation to the use of such fitting functions: what if you took an inappropriate form for bulge or disk? Many galaxies obviously pay no attention to these equations. Kent 1985 (ApJS 59,115) introduced a more general approach useful for galaxies seen strongly inclined to our line of sight. This relies on the different shapes of bulge and disk, starting with minor and major axis profiles as first estimates and generating successively closer approximations of each. Sometime a distinct small disk component (the visual lens) appears. Further, many disks are not terribly exponential in distribution - fits as a power law minus constant are just as successful, and may be more physically reasonable. Likewise, there are central regions in some late-type spirals than are closely exponential in form (sometimes called pseudobulges, in case they have a different origin). One should beware of too much interpretation of empirical equations! Besides bulge and disk components, intermediate structures (such as the visual lens) may appear. These are shown in the K-band profile of VV188=NGC 4438 (which is much messier optically than in the near-infrared). The plot is from Keel and Wehrle 1993 (AJ 106, 236), courtesy of the AAS. A pure exponential disk would be a straight line, as in the outer part of the profile. A gentle excess above this outside the bulge may be considered a lens component (or a gross departure of the disk from an exponential form). There is no basic reason that such components need be physically distinct; don't push the easy formulas too hard. Further difficulties are encountered in decomposing very late-type or dwarf systems, in which
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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Sersic showed that the exponential and de Vaucouleurs profiles may be seen as special cases of a dis

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